I think you missed the point, which is that cocaine wasn't expected there at all. Its presence indicates that there is some way for it to get into the environment that was not anticipated.

It means that now we need to figure out how it gets there, and whether or not that is a problem, and if it is how to stop it. Just because it is probably not a problem for the shrimp doesn't mean it is not a problem, because it is unlikely that whatever is releasing it into the environment does so in a way that only reaches shrimp.

It’s most likely from waste water treatment plant effluent discharged into the rivers (or upstream) in question. Those plants are typically not capable of breaking down pharmaceutical and recreational drugs, nor artificially produced hormones (such as that found in birth control pills).

The potential effect may be low for shrimp but the question is what is the effect on other organisms that consume shrimp in mass quantities and which, themselves, might ultimately end up being consumed by humans. Biomagnification has long been a concern of biologists with respect to pollutants such as DDT and mercury. [0].

[0] https://en.wikipedia.org/wiki/Biomagnification

That's typically a problem for pollutants that can't be expelled or neutralized by the body. Cocaine is organic so it probably won't have the same issue.